1. Technical Field
The invention relates to passenger seat occupant restraints and more particularly to an energy absorbing restraint.
2. Description of the Problem
Three point seat belt systems have long been used in automobiles. Three point seat belt systems combine a lap belt with a single upper torso belt crossing a person's chest from over one shoulder to a point of connection with the lap belt on the other side of the wearer's body, usually adjacent the location of a buckle located low on, or along side of, the seat.
Designers of school buses face certain difficulties in providing three point seat belts in buses. First, United States' federal regulations for passive restraints (i.e. those effective with respect to passengers whether wearing a seat belt or not) require that the rear side of a seat provide an impact barrier that bends or deforms upon impact by passengers sitting behind that seat. These requirements are codified at 49 CFR Sect. 571.222 (FMVSS 222).
The code specifies a passive restraint system for passengers, but does not require active restraints such as a two-point lap belt or three point lap/torso combination system. The passive restraint system requirements in effect provide that a compartment exist which contains an unbuckled passenger in the event of sudden deceleration. This has been met by making the back of the seat in front of a potentially unbuckled passenger into a deformable impact cushion. Although two point belt systems are offered on buses, designers need to consider three-point seat belts where requested by local or State governments, or by action groups. Although these belts are not required, when installed they too are subject to federal regulations. These requirements provide that the belts be installed in such a way as to limit a buckled passengers forward movement. The pertinent sections of the federal code are 49 CFR Sects. 571.209 and 210 (FMVSS 209 and FMVSS 210).
The conflict in requirements arises from the need to deal both with restrained and unrestrained passengers simultaneously. In a rapid deceleration event a passenger in a forward seat may be buckled in while a passenger in the seat immediately aft from the buckled in passenger may not be buckled in. With the aft passenger being spaced from the back of the forward seat, the initial response of the restraint systems occurs as a reaction to the buckled passenger moving forward in his seat and thereby applying tension on the buckled seat belt and at its attachment point to the seat frame. This can result in the belt pulling on the seat back in the forward direction and possibly reducing the strength of the seat back to meet a rear impact by the rear seat passenger. The reduction in seat back strength due to the pull on the three-point seat belt, which meets the FMVSS 210 requirement, may reduce the ability of the seat back to meet the FMVSS 222 requirements.
Recent school bus seat designs have been developed that involve a movable inner seat for the mounting of the three point seat belts and a separate seat back portion for the absorption of the impact of an unbuckled passenger. The movable inner seat was inserted into a recess within an outer, positionally fixed, but deformable, seat back. The outer seat back is then designed to deform in order to comply with FMVSS 222. Such a design is disclosed in U.S. Pat. Nos. 6,123,388, and 6,485,098. The concept of a seat inserted within a seat was also disclosed in U.S. Pat. No. 4,784,352 for an aircraft ejection seat.